Squirtle (good nic BTW)
it is a data drive as it says in the name.
It is considered "best practice" by many, myself included, to split the drive in this way, so you can store all your personal data (pics, tunes, etc) on the data drive. This means if you get a total failure of windows then you can just wipe the system drive and reinstall without losing all your personal data.
sadly windows doesn't work like this on it's own and it requires the user to actually use the data drive. Windows will put everything on the system drive as default.